That's not a slippery slope, though. That's Reddit banning subs for two different reasons. I'd they banned subs for hateful content then changed the definition of "hateful content" to include things advertisers object to then it would be. But if they're publicly stating that they're doing it because of advertisers then it's not really related.

Any time reddit says they are removing something due to "hateful content", it's just PR speak for "we got some media/advertiser backlash for this content so we are removing it".

Reddit was notoriously infested with white supremacist subs, jailbait subs, pics of dead people subs, and more, and they were all brought up to the admins many times, and the admins never took any action on this hateful content until CNN et al started writing articles about it.

It's not entirely unlike this situation with Cloudflare, really. These companies talk a big game about their principles and morals, but at the end of the day the only principle they strictly adhere to is the principle of public backlash.

How are pictures of dead people "hateful"?

Should we ban all pro-abortion subreddits as well because they promote murder of unborn children?